Western Lake Erie’s 2013 toxic algae outbreak was worse than expected, fooling the most advanced scientific prediction model the federal government has developed and covering more of the lake’s open water than any of the recent outbreaks except the 2011 record.

The University of Toledo’s top algae researcher, Tom Bridgeman, an associate professor of environmental science and a researcher at UT’s Lake Erie Center, presented a graphic that reflected that information at the UT College of Law’s 13th annual Great Lakes water-law conference on Friday.

The graphic showed this year’s bloom — while not a record-setter — went well beyond the Lake Erie islands and fanned out across more of the lake than expected. It didn’t get past Cleveland and penetrate the lake’s central basin as did the 2011 outbreak.

“The 2013 bloom was second only to 2011 in the open water,” Mr. Bridgeman told nearly 300 people who attended the seminar.

Another noteworthy feature of this year’s bloom: It was so dense along Lake Erie’s southern shoreline that a lot of it spent extended time underneath the water instead of on its surface.

High winds mixed it deep into the water. The lake’s predominant form of toxic algae, microcystis, tends to bubble up and float to the surface as it releases gases. But the mat was so thick that the weight of it kept a lot of the algae deep under water, Mr. Bridgeman said.

That helps explain why the water-treatment plant in Ottawa County’s Carroll Township, which serves 2,000 people, became so overwhelmed by the algae’s toxin, microcystin, that superintendent Henry Biggert took the unprecedented action of shutting it down. Mr. Biggert had service switched over temporarily in September to the system that serves the Port Clinton area.

That was the first time in Ohio history that a Lake Erie water-treatment plant was taken offline because of algae.

The Toledo water-treatment plant, northwest Ohio’s largest and most sophisticated, was able to neutralize the algae. But plant operators there also noticed higher-than-normal spikes and ended up getting $1 million more in emergency funds from Toledo City Council to ward off the threat.

The National Oceanic and Atmospheric Administration, using a newly developed scientific model, accurately predicted the 2013 bloom would be “significant,” but did not anticipate it being as bad as it was.

In a 110-page report planned for release later this month, a state task force trying to reduce western Lake Erie’s toxic algae will call for a 40 percent reduction in all forms of phosphorus entering northwest Ohio waterways.

The Ohio Phosphorus Task Force’s report, an update to its initial 2010 study, could affect farmers, sewage plant operators, large land-based businesses such as golf courses, and homeowners — anyone who uses or manages large amounts of fertilizers.

State and federal legislators are expected to use the task force recommendations when deciding whether to expand existing laws or adopt new ones.

Efforts could include a stronger focus on mixing nutrients in farm soil to reduce agricultural runoff into waterways, tighter controls on animal manure — including a ban on winter application — and an effort to fix sewage overflows faster.

The recommendations have been anticipated for months. They were made public by Mr. Bridgeman at the conference.

Mr. Bridgeman said the state task force chairman, Ohio Lake Erie Commission Executive Director Gail Hesse, gave him permission to release an excerpt of the report.

The seminar included discussions of similar algae problems in other parts of America, such as Florida, the Mississippi River, the Gulf of Mexico, and the Cheasapeake Bay.

“In Florida, we focus on the public-health threat,” said Monica Reimer, a Tallahassee lawyer employed by Earthjustice, one of the nation’s largest environmental law groups. “It’s not good enough to say fish are dead. Algae’s a public health threat.”

She said dozens of manatees died in the state in 2013 because they ingested toxic algae.

Emily Collins, an Ohio native who teaches at the University of Pittsburgh School of Law, likened the Cheasapeake Bay’s ecology to that of the Great Lakes.

She said people don’t realize how long it can take a system to recover once it’s been fouled: The full benefit of a 2009 executive order to clean the Cheasapeake, signed by President Obama shortly after he entered the White House, could take 20 to 40 years beyond the target date of 2025 for many of the pollution controls, Ms. Collins said.

Former Ohio Environmental Protection Agency Director Chris Korleski, who leads the U.S. EPA’s Great Lakes National Program Office in Chicago, was the keynote speaker. He said the task of restoring the Great Lakes will take decades, even with $1.3 billion allocated under the Great Lakes Restoration Initiative since 2010 to address issues such as algae and other forms of pollution, as well as invasive species.

Climate change complicates restoration efforts, Mr. Korleski said, noting that scientists now believe the greatest factor for algae is the amount of rain that falls between March 1 and June 30.

“Storms don’t feel like they did when I was a kid. They just don’t. And I don’t think that’s going to change anytime soon,” Mr. Korleski said.

“My prediction,” he added, “is we will continue to wrestle with this [algae] issue, we will continue to talk, and — over time — we will make progress.”